(1) Field of the Invention
This invention relates to a process for producing a polyester film. More particularly, it relates to a process for producing a biaxially drawn polyester film well balanced between sliding properties and transparency and having an enhanced uniformity in thickness from a substantially amorphous film predominantly comprised of polyethylene terephthalate.
(2) Description of the Prior Art
Biaxially oriented polyester films of high strength are usually produced by drawing undrawn polyester films in the longitudinal direction and then drawing the film in the transverse direction and, if required, further drawing the film in the longitudinal direction. As described in, for example, Japanese Laid-open Patent Application No. 54-8672/1979, it is known that the process conditions under which undrawn polyester films are first drawn in the longitudinal direction greatly influence the sliding properties of the resulting biaxially oriented film and the uniformity in thickness thereof. However, biaxially oriented polyester films produced by conventional processes are not completely satisfactory in sliding properties and uniformity in thickness. Furthermore, in the course of producing a high tenacity type polyester film exhibiting an extremely high strength, e.g., an F-5 value (strength at 5% elongation) of 16 kg/mm.sup.2 or more, as measured in the longitudinal direction, when the film is drawn again in the longitudinal direction, the film is liable to break and productivity is reduced.